For California lawyer and former FBI agent Richard Tosaw, summer means trekking to the Columbia River and continuing his 24-year search for the skyjacker known as D.B. Cooper. Thirty-four years ago above this southwest Washington town, Cooper parachuted from a jetliner with $200,000 -- and into folk-hero stardom. He was never seen again.

The FBI calls his crime the only unsolved skyjacking in history, and the bureau continues to keep the case open.

Tosaw (pronounced TOO-saw) believes the skyjacker's remains lie somewhere in the river. This month, he hired a team of divers to scour a stretch of the river where a small portion of the ransom money was found. It was Tosaw's second trip to the river this summer.

"I know guys who go elk hunting every year," said Tosaw, 80, who lives in the Modesto area. "I look for Mr. Cooper. It's my hobby."

It is an expensive hobby, but Tosaw, a bachelor with no children, said he "can afford to have a little fun." He paid the three-man dive team $2,500 a day for four days to search an area on the Washington side of the river about five miles west of Vancouver. The team used a barge, pushed by a tugboat, as a command center.

With the searchers wearing camera-equipped helmets, Tosaw watched what the divers saw in real time.

The river is about 400 feet across and 40 feet deep at that location. The divers concentrated on the shallows, going no deeper than 30 feet along the bank. They found all kinds of debris, including a 2,000-pound anchor believed to be about 100 years old -- but no sign of Cooper.

The hope was to find something sticking out of the silt: a leg bone or belt buckle or wallet. Tosaw said the water in the Columbia was cold enough that Cooper's body probably would be well-preserved if it was down there.

"He could also be under 5 feet of sand," Tosaw said. "It's a needle in a haystack, I know. You'd have to be a lot lucky to find him."

Tosaw has spent most of his life solving mysteries of one kind or another. As a new graduate of a Denver law school, he joined the FBI in 1951 and served as an agent for five years before starting his law practice in California. After a quarter-century, he started a business tracking down heirs of people who died with unclaimed estates.

He was never officially involved in the search for Cooper but was always intrigued by the case and became friends with some of the lead investigators. In 1981, Tosaw read a newspaper article on the 10th anniversary of the skyjacking and has been looking ever since.

"I've always liked solving mysteries, and this is a big mystery," he said. "How can a person in America in the 20th century jump from an airplane with $200,000 in ransom money and nobody knows who he is or where he is? That doesn't sit well with me. There's got to be an answer."

On his own time, he interviewed the crew and passengers of the skyjacked plane and eventually wrote and published in 1984 a book, "D.B. Cooper: Dead or Alive?" Each year, Tosaw uses the latest in high-tech equipment to search the Columbia River, which is calmest and clearest in the summer.

He has also surveyed more than 100 parachutists on whether they thought Cooper could have survived the jump; about three-fourths said it was possible if he had served in the military as a paratrooper. At that time, military service was the most likely way to have learned how to parachute. Cooper was believed to have been in his forties at the time of the skyjacking, which means he could have served in the Korean War.

The skyjacking happened on Thanksgiving eve 1971. A white man wearing a white shirt, narrow black tie, dark suit, raincoat, sunglasses and carrying a briefcase boarded Northwest Airlines Flight 305 in Portland, Ore. During the flight, he informed the crew that his briefcase contained a bomb and that he would detonate it if he was not given $200,000 in ransom money and four parachutes.

The Boeing 727 landed in Seattle, where the passengers were released and authorities complied with Cooper's demands. The plane took off for Portland with only Cooper and the crew aboard. About 45 minutes later, Cooper offered the flight attendants $2,000 each as a tip and then opened a door in the back and bailed out -- into darkness and a driving rainstorm.

That was the last anybody ever saw of Cooper. Authorities do not even know his real last name. The name he provided when he bought his ticket was Dan Cooper. After the skyjacking, a newspaper reported that police had interviewed an Oregon man named D.B. Cooper, who turned out to be the wrong man, but the name stuck.

"This is a guy who tweaked Uncle Sam's nose and appears to have gotten away with it," said retired FBI agent Ralph Himmelsbach, trying to explain Cooper's folk-hero status. Himmelsbach, who once headed the investigation, said that although Cooper broke the law, he did not hurt anyone, except probably himself.

Himmelsbach said Cooper jumped from 10,000 feet into a minus-7 degree temperature -- 69 degrees below zero with wind chill -- while wearing "a business suit and slip-on loafers. . . . It's a long shot he survived."

In February 1980, an 8-year-old boy picnicking with his family along the Columbia River found a muddy wad of $20 bills totaling $5,800. Authorities confirmed the money had been part of Cooper's loot. The find corroborated the theory that Cooper probably was dead at the bottom of the river, but others speculated that he was clever enough to have placed the money in the river as a diversion.

It is near this spot -- an area where debris naturally collects -- that Tosaw has concentrated his efforts.

"People want to believe he got away with it. They want to believe he's alive somewhere," Tosaw said. "I just want to find his wallet, so the world will know who the hell D.B. Cooper really was."